Physical parameters of lambda Bootis stars

This is the first of two papers whose main goal is to update and improve th e information available on the physical properties of the lambda Bootis sta rs. The determination of the stellar parameters is of fundamental importanc e to shed light into the different theories proposed to explain the lambda Bootis phenomenon. With this aim, projected rotational velocities, effectiv e temperatures, surface gravities and chemical abundances of a sample of su spected lambda Bootis stars have been calculated. Five objects showing comp osite spectra typical of binary systems were found in our analysis. The abu ndance distribution of the program stars does not resemble the chemical com position of the class prototype, lambda Boo, which poses some concerns rega rding the idea of a well-defined, chemically homogeneous group of stars. A possible relation between rotational velocities and the lambda Bootis pheno menon has been found. This result would be in agreement with the accretion scenario proposed by Turcotte & Charbonneau (1993).