Photometry of slow X-ray pulsars. III - The GX 17 + 2 candidate

The first photometry of the faint candidate star for the optical counterpart of the intense x-ray source GX 17+2 (= 3U 1813--14) is presented. This source is of particular interest because of its associated radio emission, and the existence of a 1913 s periodic modulation of the x-ray flux, which makes GX 17+2 the slowest x-ray pulsar yet observed. Photoelectric photometry on three nights yields V = 17.51, (B--V) = 1.26, (U--B) = 1.03, with no evidence for variabiliy; a limit of 0.05 mag half-amplitude is established on any optical analog of the x-ray period. This limit is interpreted to yield information on the geometry of the GX 17+2 system. Although this object is often thought to resemble closely low-mass x-ray binary systems such as Sco X-1, which has a 19 hour orbital period, these results indicate that any binary period in GX 17+2 is very likely longer than 2 days, exeeding that of all the known low-mass x-ray systems. 1 figure.