  We study the interaction of two successive coronal mass ejections (CMEs)
during the 2010 August 1 events using STEREO/SECCHI COR and HI data. We obtain
the direction of motion for both CMEs by applying several independent
reconstruction methods and find that the CMEs head in similar directions. This
provides evidence that a full interaction takes place between the two CMEs that
can be observed in the HI1 field-of-view. The full de-projected kinematics of
the faster CME from Sun to Earth is derived by combining remote observations
with in situ measurements of the CME at 1 AU. The speed profile of the faster
CME (CME2; ~1200 km/s) shows a strong deceleration over the distance range at
which it reaches the slower, preceding CME (CME1; ~700 km/s). By applying a
drag-based model we are able to reproduce the kinematical profile of CME2
suggesting that CME1 represents a magnetohydrodynamic obstacle for CME2 and
that, after the interaction, the merged entity propagates as a single structure
in an ambient flow of speed and density typical for quiet solar wind
conditions. Observational facts show that magnetic forces may contribute to the
enhanced deceleration of CME2. We speculate that the increase in magnetic
tension and pressure, when CME2 bends and compresses the magnetic field lines
of CME1, increases the efficiency of drag.
