Lithium and rotation on the subgiant branch I. Observations and spectral analysis

We have obtained new high resolution spectroscopic observations of the lith ium line at 6707.81 Angstrom and derived lithium abundances (A(Li)) by spec tral synthesis for a sample of about 120 F-, G- and It-type Population I su bgiant stars. For each of these stars, high precision rotational velocity o btained with the CORAVEL spectrometer is available. We present the behavior of the lithium abundance as a function of effective temperature, which sho ws a sort of discontinuity around 5600 K, somewhat later than the well know n rotational discontinuity.