  Colliding wind binaries (CWBs) have been considered as a possible high energy
$\gamma$-ray sources for some time, however no system other than $\eta$ Car has
been detected. In the paper a sample of seven CWBs (WR 11, WR 70, WR 137, WR
140, WR 146, WR 147) which, by means of theoretic modelling, were deemed most
promising candidates, was analyzed using almost 7 years of the Fermi-LAT data.
WR 11 ($\gamma^2$ Vel) was detected at 6.1$\sigma$ confidence level with a
photon flux in 0.1-100 GeV range
$(1.8\pm0.6)\times10^{-9}~\mathrm{ph~cm^{-2}~s^{-1}}$ and an energy flux
$(2.7\pm0.5)\times10^{-12}~~\mathrm{erg~cm^{-2}~s^{-1}}$. At the adopted
distance $d=340$ pc this corresponds to a luminosity
$L=(3.7\pm0.7)\times10^{31}~\mathrm{erg~s^{-1}}$. This luminosity amounts to
$\sim6\times10^{-6}$ fraction of the total wind kinetic power and
$\sim1.6\times10^{-4}$ fraction of the power injected into the wind-wind
interaction region of this system. Upper limits were set on the high energy
flux from the WR 70 and WR 140 systems.
