  Determinations of beryllium abundance in stars, together with lithium,
provide a key tool to investigate the so far poorly understood extra-mixing
processes at work in stellar interiors. We measured Be in three open
clusters,complementing existing Be surveys, and aiming at gathering a more
complete empirical scenario of the evolution of Be as a function of stellar age
and temperature. Specifically, we analyzed VLT/UVES spectra of members of NGC
2516, the Hyades, and M 67 to determine their Be and Li abundances. In the
first two clusters we focused on stars cooler than 5400 K, while the M 67
sample includes stars warmer than 6150 K, as well as two subgiants and two blue
stragglers. We also computed the evolution of Be for a 0.9 Mo star based on
standard evolutionary models. We find different emprical behaviours for stars
in different temperature bins and ages. Stars warmer than 6150 K show Be
depletion and follow a Be vs. Li correlation while Be is undepleted in stars in
the ~6150-5600 K range. NGC 2516 members cooler than 5400 K have not depleted
any Be, but older Hyades of similar temperature do show some depletion. Be is
severely depleted in the subgiants and blue stragglers. The results for warm
stars are in agreement with previous studies, supporting the hypothesis that
mixing in this temperature regime is driven by rotation. The same holds for the
two subgiants that have evolved from the "Li gap". This mechanism is instead
not the dominant one for solar-type stars. We show that Be depletion of cool
Hyades cannot simply be explained by the effect of increasing depth of the
convective zone. Finally, the different Be content of the two blue stragglers
suggests that they have formed by two different processes (i.e., collisions vs.
binary merging).
