  The dissertation presents result from study of Pre-main sequence (PMS) stars
that are in the earliest stages of stellar evolution. These young stellar
objects are still in the process of formation, and the energy they emit is
produced only by gravitational contraction. The main results were obtained with
the telescopes at the National Astronomical Observatory Rozhen, as well as with
the use of archival photographic observations and spectral observations
obtained in collaboration with colleagues from abroad. Our results are obtained
by studying the photometric and spectral variability of PMS stars. Our main
goal is to study the processes of star formation, the formation of
circumstellar disks, the structure of the circumstellar environment and the
interaction of the star-disk system. The results of our research have been
published in 65 scientific papers that have been cited over 300 times.
  The photometric and spectral variability of PMS stars is of great importance
for modeling star formation processes. On the one hand, photometric variability
allows us to easily detect young objects, since they are characterized by rapid
variability and in many cases with large amplitudes. On the other hand, stars
form in groups and are physically located in the same geometric space, and
several variable objects that are at approximately the same stage of evolution
can be observed simultaneously. Comparing star systems of different ages can be
used to trace the stages of stellar evolution.
