The study of the first stages in the formation of stars is one of the currently most active research fields in stellar astronomy. Although much progress has been made since the discovery of T Tauri stars in 1942 several questions still remain open. There are basically two classes of problems that need to be assessed: the detailed structure and evolution of single stars from the protostellar phase to the main sequence, and the nature of the processes that determine and regulate the onset of star-formation within molecular clouds in the first place. Pre-main sequence (PMS) stars are characterized by a high degree of activity like winds, jets, outflows etc. They are interacting with the circumstellar environment in which they are embedded. Mostly they have strong infrared and/or UV excesses and show photometric and spectroscopic variability on time scales from minutes to years. On time scales of weeks to hours they are generated by variable extinction due to cirumstellar dust, by clumped accretion and chromospheric activity. On time scales of half an hour to some hours the variation may be due to pulsation if the star lies in or near the instability strip. Stars over essentially the whole mass spectrum can become pulsationally unstable during various stages of their evolution. The fact that young stars during their evolution to the main sequence move across the instability region raises the possibility that at least part of the observed stellar activity could also be due to stellar pulsations. One method to separate the different contaminations (from reflection nebulae, aligned interstellar dust, gas and dust shells...) of the intrinsic star light is to study the variability (if any) in all the observable physical parameters as a function of time. The discovery of pulsation in PMS stars is extremely important, since it provides a unique means for constraining the internal structure of young stars and for testing evolutionary models. Although it is difficult to put constraints on the characteristics of young stars to define whether they are in their PMS phase of evolution or not, two different types of (possible) PMS objects can be identified:

• T Tauri stars are newly formed low-mass stars that have recently become visible in the optical range. Joy 1942 first discovered this group of stars in the Taurus-Auriga dark cloud and named them after their brightest member 'T Tauri'. They are PMS stars of late spectral type (G, K and M). They show apparently normal photospheres overlain by continuum and line-emission characteristics of a hotter (about 7000 K to 10000 K) envelope. T Tauri stars display irregular and large light variations and are associated with dark or bright nebulae from which they were born. They are often found in connection with groups of OB stars, the O-associations and show strong IR and/or UV excesses.
  As they are of late spectral types they do not provide the necessary scenario for the Kappa mechanism for driving pulsation.
  

• In 1960 Herbig suggested that 'Be and Ae stars associated with nebulosity' are PMS stars of intermediate mass (2 - 10 solar masses) in their radiative phase of contraction onto the main sequence. Thé et al (1994) undertook the task to build a catalog of known Herbig Ae/Be (HAEBE) stars defining the following characteristics: they are a collection of hot emission line stars of spectral type B, A or F and they always have a near- or far-IR excess. Besides many of these objects have surface gravities appropriate to PMS stars which would place them above the main sequence and confirm their pre-main sequence nature. These results do not prove that every Be/Ae star is in the PMS phase of evolution.

Pulsation In Pre-Main Sequence Stars

Most of the studies of variability in PMS stars have examined the large-amplitude photometric fluctuations and thus have investigated the behaviour of the obscuring medium which causes the fluctuations. Only few studies have been devoted to the photometric variations on shorter time scales and smaller amplitudes in order to detect delta Scuti like pulsation (Breger 1972, ApJ 171, 539; Kurtz & Marang 1995, MNRAS 276, 191; etc.). 'Regular' delta Scuti stars are slightly evolved, have spectral types A to F, belong to luminosity classes V to III and lie in the classical instability strip. They pulsate in radial and non-radial modes with periods between 30 minutes and 8 hours. As a mature delta Scuti star differs from a young one mostly in the inner regions (central density) and in its higher luminosity (see Fig.1), the discovery of pulsation could potentially provide constraints on the internal structure of an acknowledged pre-main sequence star.

Fig.1: Internal density profile (normalized to the central value) of a 3 M_sun star in different evolutionary stages. Upper curve: A protostar on the birthline; middle curve: PMS star with L = 55 L_sun, T_eff = 6900K, R = 5.3 R_sun; post-MS star with the same T_eff but L = 135 L_sun and R = 8.3 R_sun. (Figure from Marconi & Palla, 1998, ApJ 507, L141)

In 1995 Kurtz & Marang (MNRAS 276, 191) discovered delta Scuti pulsation with a period of 4.99 hours in the well-studied pre-main sequence Herbig Ae star HR 5999 (spectral type A7 III-IVe), which is embedded in an obscured region of Scorpius with many T Tauri stars in the associated dark cloud. The observations clearly show periodic light variations with a visual amplitude of 0.013 mag (peak-to-peak) in the presence of 0.35 mag of non-periodic variability caused by variations in the obscurations by dust in the disc surrounding HR 5999, which confirms the PMS nature of this star.
Marconi and Palla (1998, ApJ, 507, 141) investigated the pulsational properties of PMS stars by means of linear and nonlinear calculations and tried to define the instability strip for these stars in the HR diagram (Fig.2). They compared their results with the observations of HR 5999 and also identified possible candidates for pulsational variability among known Herbig Ae/Be stars that are located within or close to the instability strip boundaries. Despite the relatively short time spent by PMS stars inside the strip, meanwhile three of the candidate PMS pulsators have been confirmed to really pulsate, i.e. HD 104237 (Kurtz & Müller 1999, MNRAS 310, 1071), HD 35929 and V 351 Ori (Marconi et al., 200 0, AA 355, 35). As up to now only 9 pulsating PMS stars are known, it is very important to increase this number in order to be able to define the PMS instability strip observationally and provide additional constraints for its theoretical definition.

Fig.2: Left: Location of the instability strip of PMS stars in the HR diagram.
Right: Distribution of known PMS stars within or near the boundaries of the instability strip that can be considered candidates for pulsational instability; pentagon: HR 5999; triangles and squares: 13 PMS candidate pulsators.(Figure from Marconi & Palla, 1998, ApJ 507, L141)

Timeseries Photometry Of Young Open Clusters

The detection of PMS stars as members of young clusters has been the subject of several works in the past and has some important advantages:

• As the age and mass of the cluster members is well-determined all observational quantities can be determined more reliable than for individual stars.
• All cluster members are located at about the same distance.
• The foreground interstellar extinction can be determined using early type members (which are undoubtedly true cluster members). This is necessary since the extinction in the direction of PMS objects is due to foreground as well as circumstellar material and the latter often obeys a different extinction law than the former.
• All cluster members are young, therefore confusion with evolved objects is not to be expected.