  Coronal mass ejections (CMEs) may disturb the solar wind either by overtaking
it, or by expanding into it, or both. CMEs whose front moves faster in the
solar wind frame than the fast magnetosonic speed, drive shocks. Such shocks
are important contributors to space weather, by triggering substorms,
compressing the magnetosphere and accelerating particles. In general, near 1
AU, CMEs with speed greater than about 500 km s$^{-1}$ drive shocks, whereas
slower CMEs do not. However, CMEs as slow as 350 km s$^{-1}$ may sometimes,
although rarely, drive shocks. Here, we study these slow CMEs with shocks and
investigate the importance of CME expansion in contributing to their ability to
drive shocks and in enhancing shock strength. Our focus is on CMEs with average
speeds under 375 km s$^{-1}$. From Wind measurements from 1996 to 2016, we find
22 cases of such shock-driving slow CMEs, and, for about half of them (11 out
of the 22), the existence of the shock appears to be strongly related to CME
expansion. We also investigate the proportion of all CMEs with speeds under 500
km s$^{-1}$ with and without shocks in solar cycles 23 and 24, depending on
their speed. We find no systematic difference, as might have been expected on
the basis of the lower solar wind and Alfv\'en speeds reported for solar cycle
24 vs. 23. The slower expansion speed of CMEs in solar cycle 24 might be an
explanation for this lack of increased frequency of shocks, but further studies
are required.
