  Aims. We report the discovery of a peculiar object observed serendipitously
with XMM-Newton. We present its timing and spectral properties and investigate
its optical counterpart. Methods. The light curve of the X-ray source, its
spectrum, and the spectrum of the best optical counterpart are presented and
analyzed. Results. The X-ray flux decreases by a factor of 6.5 within 1 h and
stays in a low state for at least 10 h, thereby suggesting the presence of an
eclipse. The spectrum is very soft, a power law with a slope of Gamma~2.8, and
does not change significantly before and after the flux drop. The source is
spatially coincident within few arc-seconds with a Seyfert~2 galaxy belonging
to a galaxy pair. Conclusions. Although the background AGN seems the best
counterpart, neither the temporal nor the spectral properties of the X-ray
source are compatible with it. We investigate the possibility of having a
foreground low-mass X-ray binary in quiescence, where the companion is not
detected in the optical wavelength.
