The Seyfert galaxy NGC 6814 is unique in showing evidence of a periodicity in its X-ray light curve, first seen in EXOSAT data and then confirmed by a Ginga observation. A further Ginga light curve of this source taken 18 months later shows that the periodic signal is still present. The longevity and stability of the phenomenon suggest that the hypothesized central supermassive black hole might have a low-mass satellite. If this is a star that does not fill its Roche lobe, angular momentum losses should lead to a period decrease on a time-scale <10 3 yr, incompatible with the limits on P˙ derived from the X-ray data. We show instead that Roche-lobe overflow of an orbiting star naturally produces both the accretion flow required to power the nucleus and the observed period behaviour