  In this paper we measure the merger fraction and rate, both minor and major,
of massive early-type galaxies (M_star >= 10^11 M_Sun) in the COSMOS field, and
study their role in mass and size evolution. We use the 30-band photometric
catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey,
to define close pairs with a separation 10h^-1 kpc <= r_p <= 30h-1 kpc and a
relative velocity Delta v <= 500 km s^-1. We measure both major (stellar mass
ratio mu = M_star,2/M_star,1 >= 1/4) and minor (1/10 <= mu < 1/4) merger
fractions of massive galaxies, and study their dependence on redshift and on
morphology. The merger fraction and rate of massive galaxies evolves as a
power-law (1+z)^n, with major mergers increasing with redshift, n_MM = 1.4, and
minor mergers showing little evolution, n_mm ~ 0. When split by their
morphology, the minor merger fraction for early types is higher by a factor of
three than that for spirals, and both are nearly constant with redshift. Our
results show that massive early-type galaxies have undergone 0.89 mergers (0.43
major and 0.46 minor) since z ~ 1, leading to a mass growth of ~30%. We find
that mu >= 1/10 mergers can explain ~55% of the observed size evolution of
these galaxies since z ~ 1. Another ~20% is due to the progenitor bias (younger
galaxies are more extended) and we estimate that very minor mergers (mu < 1/10)
could contribute with an extra ~20%. The remaining ~5% should come from other
processes (e.g., adiabatic expansion or observational effects). This picture
also reproduces the mass growth and velocity dispersion evolution of these
galaxies. We conclude from these results that merging is the main contributor
to the size evolution of massive ETGs at z <= 1, accounting for ~50-75% of that
evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is
related to minor (mu < 1/4) events.
