Detection of differential rotation in psi Cap with profile analysis

We report detection of differential rotation on the F5 dwarf psi Cap using line profile analysis. The Fourier transform of both Fe I lambda 5775 and S i I lambda 5772 are used to obtain a projected rotational velocity of v sin i = 42 +/- 1 km s(-1). Modelling of the Fourier transformed profiles shows that the combined effects of equatorial velocity, inclination and differen tial rotation dominate the line profile while limb darkening and turbulence velocities have only minor effects. Rigid rotation is shown to be inconsis tent with the measured profiles. Modelling the line profiles analogous to s olar differential rotation we find a differential rotation parameter of alp ha = 0.15 +/- 0.1 (15 +/- 10%) comparable to the solar case. To our knowled ge this is the first successful measurement of differential rotation throug h line profile analysis.