  Context: Metric type II bursts are the most direct diagnostic of shock waves
in the solar corona.
  Aims: There are two main competing views about the origin of coronal shocks:
that they originate in either blast waves ignited by the pressure pulse of a
flare or piston-driven shocks due to coronal mass ejections (CMEs). We studied
three well-observed type II bursts in an attempt to place tighter constraints
on their origins.
  Methods: The type II bursts were observed by the ARTEMIS radio spectrograph
and imaged by the Nan\c{c}ay Radioheliograph (NRH) at least at two frequencies.
To take advantage of projection effects, we selected events that occurred away
from disk center.
  Results: In all events, both flares and CMEs were observed. In the first
event, the speed of the shock was about 4200 km/s, while the speed of the CME
was about 850 km/s. This discrepancy ruled out the CME as the primary shock
driver. The CME may have played a role in the ignition of another shock that
occurred just after the high speed one. A CME driver was excluded from the
second event as well because the CMEs that appeared in the coronagraph data
were not synchronized with the type II burst. In the third event, the
kinematics of the CME which was determined by combining EUV and white light
data was broadly consistent with the kinematics of the type II burst, and,
therefore, the shock was probably CME-driven.
  Conclusions: Our study demonstrates the diversity of conditions that may lead
to the generation of coronal shocks.
