  Cosmic shear is a key probe of modern cosmology. Amongst its challenges are
shape noise and intrinsic alignments, both due to our ignorance of the unlensed
shape of the source galaxies. I argue here that Einstein rings may be used as
standard shapes to measure the external shear along their line of sight. In the
Euclid era, this new observable is expected to be a competitive and
complementary probe of the large-scale structure of the Universe.
