  Multiple generations of stars are routinely encountered in globular clusters
but no convincing evidence has been found in Galactic open clusters to date. In
this paper we use new photometric and spectroscopic data to search for multiple
stellar population signatures in the old, massive open cluster, Melotte~66. The
cluster is known to have a red giant branch wide in color, which could be an
indication of metallicity spread. Also the main sequence is wider than what is
expected from photometric errors only. This evidence might be associated with
either differential reddening or binaries. Both hypothesis have, however, to be
evaluated in detail before recurring to the presence of multiple stellar
populations. New, high-quality, CCD UBVI photometry have been acquired to this
aim with high-resolution spectroscopy of seven clump stars, that are
complemented with literature data. Our photometric study confirms that the
width of the main sequence close to the turn off point is entirely accounted
for by binary stars and differential reddening, with no need to advocate more
sofisticated scenarios, such as metallicity spread or multiple main sequences.
By constructing synthetic color-magnitude diagrams, we infer that the binary
fraction has to be as large as 30$%$ and their mass ratio in the range 0.6-1.0.
As a by-product of our simulations, we provide new estimates of the cluster
fundamental parameters. We measure a reddening E(B-V)=0.15$\pm$0.02, and
confirm the presence of a marginal differential reddening. The distance to the
cluster is $4.7^{+0.2}_{-0.1} $kpc and the age is 3.4$\pm$0.3 Gyr, which is
somewhat younger and better constrained than previous estimates. Our detailed
abundance analysis reveals that, overall, Melotte~66 looks like a typical
object of the old thin disk population.
