NGC 1893: a young open cluster rich in multitype variable stars

In this work, we have studied the variable stars in the young open cluster NGC 1893 based on a multiyear photometric survey covering a sky area around the cluster up to 31 arcmin x 31 arcmin wide. More than 23 000 images in the V band taken from 2008 January to 2017 February with different telescopes, complemented with 90 images in the B band in 2014 and 2017, were reduced, and light curves were derived in V for 5653 stars. By analysing these light curves, we detected 147 variable stars (85 of them being new discoveries), including 110 periodic variables, 15 eclipsing binaries, and 22 non-periodic variables. Proper motions, radial velocities, and colour-magnitude and two-colour diagrams were used to identify the cluster membership of these variable stars, resulting in 84 members. Periodic variable members were then classified into different variability types, mainly according to their magnitudes and to their periods of variability, as well as to their positions in the Hertzsprung-Russell diagram for the early-type stars. As a result, among main-sequence periodic variable members, we identified five beta Cep candidates, seven slowly pulsating B-type candidates, and thirteen fast-rotating pulsating B-type (FaRPB) candidates (one of which is a confirmed classical Be star). While most of the FaRPB stars display properties similar to the ones discovered in NGC 3766 by Mowlavi et al., five of them have periods below 0.1 d, contrary to expectations. Additional observations, including spectroscopic, are called for to further characterize these stars. We also find a binary candidate harbouring a delta-Scuti candidate.