Wesselink radii of double-mode cepheids

The pulsation properties of double-mode cepheids are discussed and found to be indistinguishable from those of normal Population I cepheids. Intrinsic colours of double-mode cepheids show that there is no preferred temperature domain for these stars. Simultaneous BVRI photometry and photoelectric radial velocities are used to estimate the radii of eight double-mode cepheids by a modified Wesselink technique. The period-radius relationship is found to be the same as that of normal cepheids, but very different from predictions of pulsation theory using chemically homogeneous stars. (author)