Evidence for a 2-h optical modulation in GS 1826-24

We report the discovery of a 2.1-h optical modulation in the transient source GS 1826 - 24, based on two independent, high-time resolution photometric observing runs. There is additional irregular variability on shorter time-scales. The source also exhibited an optical burst during each observation, with peak fluxes consistent with those of the three X-ray bursts so far detected by BeppoSAX. We compare the low-amplitude variation (similar to 0.06 mag) to that seen on the orbital periods of the short-period X-ray bursters, X1636 - 536 and X1735 - 444, as well as the similarity in their non-periodic fluctuations. Other transient neutron star LMXBs possess short periods in the range 3.8-7.1 h. However, if confirmed as the orbital, a 2.1-h modulation would make GS 1826 - 24 unique and therefore of great interest within the context of LMXB formation and evolution.