A CCD search for variable stars of spectral type B in the Northern Hemisphere open clusters. V. NGC2169

We present results of a search for variable stars in the field of the young open cluster NGC 2169. The General Catalogue of Variable Stars (GCVS, http://www.sai.msu.su/groups/cluster/gcvs/) lists four variable stars in the field we observed, viz., two beta Cep stars, V 916 and V 917 Ori, an alpha(2) CVn variable, V 1356 Ori, and an RRc variable, V 1154 Ori. We find V 916 and V 1154 Ori to be constant in light. We confirm the variability of V 917 Ori, but not the period given in GCVS. For the chemically peculiar A0V Si star V 1356 Ori we definitely establish the period of 1.565 d, thus settling the uncertainty persisting in the literature since the star was discovered to be variable. In addition, we find two other stars to be variable in light. Both show irregular variations. For V 917 Ori, one of the two GCVS P Cep variables, we determine a period of 0.267 d (frequency 3.7477 d(-1)). However, prewhitening with this period leaves a significant amount of the star's light-variation unaccounted for. Since the star shows emission at Halpha, we hypothesize that the unaccounted for variation is caused by an erratic, Be-type activity. As to the periodic term, we consider three hypotheses: (1) beta Cep-type pulsation, (2) rotational modulation of the lambda Eri type, and (3) ellipsoidal variation due to distorted primary component in a close binary system. After deriving the star's effective temperature from Stromgren indices and the luminosity from the distance modulus of the cluster, we show that while the third hypothesis is untenable, the first two should be retained. However, neither is entirely satisfactory. For a number of stars we provide the V magnitudes. For 14 brightest stars in our field we also obtain the photometric alpha -index, a measure of the Halpha equivalent width. From the a index, we detect mild emission at Halpha in two stars, V 917 Ori and NGC2169-8.