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On the ultracompact nature of 4 U 1822 – 000

We report the discovery of a periodic modulation in the optical lightcurve of the candidate ultra-compact X-ray binary 4U 1822–000. 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 4U 1822–000 to be 6.3 kpc. The long orbital period and casts severe doubts on the ultra-compact nature of 4U 1822–000.