CONSTRAINING THE TRUE NATURE OF AN EXOTIC BINARY IN THE CORE OF NGC 6624

We report on the identification of the optical counterpart to Star1, the exotic object serendipitously discovered by Deutsch et al. in the core of the Galactic globular cluster NGC 6624. Star1 has been classified by Deutsch et al. as either a quiescent cataclysmic variable or a low-mass X-ray binary. Deutsch et al. proposed StarA as a possible optical counterpart to this object. We used high-resolution images obtained with the Hubble Space Telescope to perform a variability analysis of the stars close to the nominal position of Star1. While no variability was detected for StarA, we found another star, referred to here as COM_Star1, showing a clear sinusoidal light modulation with amplitude Δm F435W ~ 0.7 mag and an orbital period of P orb ~ 98 minutes. The shape of the light curve is likely caused by strong irradiation by the primary heating of one hemisphere of the companion, thus suggesting a quite hot primary.