  We have investigated the 2-D stellar distribution in the outer parts of three
nearby open clusters: NGC 2287 (=M41), NGC 2516, and NGC 2548 (=M48).
Wide-field star counts have been performed in two colours on pairs of digitized
ESO and SRC Schmidt plates, allowing us to select likely cluster members in the
colour-magnitude diagrams. Cluster tidal extensions were emphasized using a
wavelet transform. Taking into account observational biases, namely the galaxy
clustering and differential extinction in the Galaxy, we have associated these
stellar overdensities with real open cluster structures stretched by the
galactic gravitational field. As predicted by theory and simulations, and
despite observational limitations, we detected a general elongated (prolate)
shape in a direction parallel to the galactic Plane, combined with tidal tails
extended perpendicularly to it. This geometry is due both to the static
galactic tidal field and the heating up of the stellar system when crossing the
Disk. The time varying tidal field will deeply affect the cluster dynamical
evolution, and we emphasize the importance of adiabatic heating during the
Disk-shocking. In the case of NGC 2548, our dating of the last shocking with
the Plane (based on a tidal clump) is consistent with its velocity. During the
10-20 Z-oscillations experienced by a cluster before its dissolution in the
Galaxy, crossings through the galactic Disk contribute to at least 15% of the
total mass loss. Using recent age estimations published for open clusters, we
find a destruction time-scale of about 600 Myr for clusters in the solar
neighbourhood.
