  We have studied the effect of cobalt substitution in
$L1_{0}$-Fe$_{1-x}$Co$_{x}$Pt films by means of x-ray magnetic circular
dichroism (XMCD) and first-principles calculations. The magnetic moments of Fe
($\sim$2.5 $\mu_{\rm B}$) and Co ($\sim$1.5 $\mu_{\rm B}$) deduced using XMCD
were almost unchanged upon Co doping, and the net magnetization decreases with
increasing Co content. Calculation also showed that the 3$d$ electrons that
have been added by Co substitution occupy only spin-down bands.
