  We study symmetry breaking and topological defects in a supersymmetric model
with gauge group $\text{U}(2)$, which can be identified with the right-handed
part $\text{SU}(2)_R \times \text{U}(1)_{B-L}$ of an extended electroweak
symmetry of the Standard Model. The model has two phases of hybrid inflation
terminated by tachyonic preheating where either monopoles and strings or,
alternatively, dumbbells are formed. In the first case a stochastic
gravitational wave background is predicted in the LIGO-Virgo band, possibly
extending to the LISA frequency band and to nanohertz frequencies, which is
generated by a metastable cosmic string network. In the second case no
topological defects survive inflation and no stochastic gravitational wave
background is produced.
