  We report on our search for very-long-term variability (weeks to years) in
X-ray binaries (XRBs) in the giant elliptical galaxy M87. We have used archival
Chandra imaging observations to characterise the long-term variability of 8 of
the brightest members of the XRB population in M87. The peak brightness of some
of the sources exceeded the ultra luminous X-ray source (ULX) threshold
luminosity of ~ 10^{39} erg/s, and one source could exhibit dips or eclipses.
We show that for one source, if it has similar modulation amplitude as in
SS433, then period recoverability analysis on the current data would detect
periodic modulations, but only for a narrow range of periods less than 120
days. We conclude that a dedicated monitoring campaign, with appropriately
defined sampling, is essential if we are to investigate properly the nature of
the long-term modulations such as those seen in Galactic sources.
