Rotational velocity of the cool component of Capella from differential speckle interferometry

Results of differential speckle interferometry of Capella are reported. The observations were performed with a photon-counting imaging system attached to the Main stellar spectrograph of the 6-m telescope in a narrow spectral. range containing the Na D1 and D2 doublet lines. Since the star is a binary, the image photocenter is displaced by an amount epsilon(lambda) which depends on the angular separation between the components and their magnitude difference at the wavelength lambda. With the components of Capella aligned along the spectrograph slit, the differential speckle displacement in the Na lines reached 0.'' 025. The error in epsilon(lambda) due to photon noise, optical aberrations, anti geometrical distortions of the detector was 0.'' 001. By combining the function epsilon(lambda) with the total spectrum of Capella, we extracted the spectrum of the cooler component and determined its rotational velocity, vsini = 8.5 +/- 1 km s(-1). It is concluded that both stars in the system are asynchronous rotators.