The angular resolution for ground-based observations is limited by the atmospherie turbulence which deforms randomly the light's wavefront of an astronomical object. Adaptive optics has been developed to correct this deformation in real time. The correction, however, is often only partial and deconvolution of the image is important in order to do accurate photometry. The first part of this thesis presents the estimation of the point spread function for the adaptive optics system ADONIS of the 3.6m ESO telescope. The variability of the atmospheric conditions makes it difficult to calibrate it properly by observing a point-like source. For this reason, I used the method developed for the adaptive optics system PUEO which estimates the point spread function from the wavefront measurements (based on a curvature sensor), and I adapted it for the ADONIS system (based on a Shack-Hartmann sensor). I applied the method to data obtained under different atmospheric conditions and discuss its limits. In particular, the model cannot reproduce a certain variable aberration. Its origin is probably due to the presence of a local non-stationary turbulence. This turbulence is clearly put to evidence from analyzing the spatial and temporal properties of the residual phase. In the second part of this thesis, I present and discuss the results of observations on the photometric variability of YY Orionis stars. These are young low-mass stars showing signatures of accreting material in their spectra. For the star YY Ori, a period could be determined in its light curve. l interpret this variability as the presence of a large spot on the stellar surface appearing and disappearing as the star rotates.
