SIMBAD references

Photometric UBVRI observations of the YY Orionis stars YY Ori, SY Ori, CE Ori, NS Ori and XX Ori are presented. The luminosity of all stars is variable on a timescale of a few days. This variation is rather irregular and no periodic variability was observed in the present data sets. The V-I and V-R colors become redder when the stars become fainter, suggesting that inhomogeneous surface temperature distributions are responsible for the observed irregular variability. Two stars located in the vicinity of our program stars and observed in the same frames turned out to be variable. They were identified as V481 Ori and AO Ori. We report a periodic modulation of the V481 Ori light curve with a period of 4.9 days.