  We present an analysis of long-term X-ray and optical observations of the
Wolf-Rayet binary WR 25. Using archival data from observations with the
XMM-Newton and the Swift observatories spanning over ~10 yr, we show that WR 25
is a periodic variable in X-rays with a period of $208 \pm 3$ days. X-ray light
curves in the 0.5-10.0 keV energy band show phase-locked variability, where the
flux increased by a factor of ~2 from minimum to maximum, being maximum near
periastron passage. The light curve in the soft energy band (0.5-2.0 keV) shows
two minima indicating the presence of two eclipses. However, the light curve in
the hard energy band (2.0-10.0 keV) shows only one minimum during the apastron
passage. The X-ray spectra of WR 25 were explained by a two-temperature plasma
model. Both the cool and the hot plasmas were constant at 0.628+/-0.008 and
2.75+/-0.06 keV throughout an orbital cycle, where the cooler plasma could be
due to the small scale shocks in a radiation-driven outflow and the high
temperature plasma could be due to the collision of winds. The column density
varied with the orbital phase and was found to be maximum after the periastron
passage, when the WN star is in front of the O star. The abundances of WR 25
were found to be non-solar. Optical V-band data of WR 25 also show the
phase-locked variability, being at maximum near periastron passage. The results
based on the present analysis indicate that WR 25 is a colliding wind binary
where the presence of soft X-rays is attributed to individual components;
however, hard X-rays are due to the collision of winds.
