B-type pulsators in the open cluster NGC 884 (χ Persei).

We present results of the photometric CCD search for B-type pulsators in the central region of the χ Persei cluster. The most interesting result is the discovery of two β Cephei stars, Oo 2246 and Oo 2299, the first ones found in χ Persei. Oo 2246 pulsates with two periods equal to 0.184 and 0.171 d. The light of the other β Cephei star, Oo 2299, varies with a single period of about 0.318 d. This is one of the longest periods known for this type of variable. The photometric Q indices of the two β Cephei stars fall in the middle of the range covered by all β Cephei variables. In the HR diagram, the position of Oo 2246 and Oo 2299 coincide with the low-temperature limit of the β Cephei instability strip in NGC 3293. The pulsational properties of both β Cephei stars in χ Persei are in agreement with the main sequence stage of evolution. In addition to the β Cephei stars, we found nine other variables, including two eclipsing stars, Oo 2301 and Oo 2311. Almost all Be stars in the field turned out to be variable. In the cluster's colour-magnitude diagram, they appear to be slightly redder than non-Be stars of similar spectral type.