On the ultra-compact nature of 4U1822-000

We report the discovery of a periodic modulation in the optical lightcurve of the candidate ultra-compact X-ray binary 4U1822-000. Using time-resolved optical photometry taken with the William Herschel Telescope we find evidence for a sinusoidal modulation with a semi-amplitude of 8 percent and a period of 191 min, which is most likely close to the true orbital period of the binary. Using the van Paradijs & McClintock relation for the absolute magnitude and the distance modulus allowing for interstellar reddening, we estimate the distance to 4U1822-000 to be 6.3 kpc. The long orbital period and casts severe doubts on the ultra-compact nature of 4U1822-000.