  Context. During their journey to perihelion, comets may appear in the
field-of-view of space-borne optical instruments, showing in some cases a
nicely developed plasma tail extending from their coma and exhibiting an
oscillatory behaviour. Aims. The oscillations of cometary tails may be
explained in terms of vortex shedding because of the interaction of the comet
with the solar wind streams. Therefore, it is possible to exploit these
oscillations in order to infer the value of the Strouhal number $St$, which
quantifies the vortex shedding phenomenon, and the physical properties of the
local medium. Methods. We used the Heliospheric Imager (HI) data of the Solar
TErrestrial Relations Observatory (STEREO) mission to study the oscillations of
the tails of the comets 2P/Encke and C/2012 S1 (ISON) during their perihelion
in Nov 2013, determining the Strouhal numbers from the estimates of the halo
size, the relative speed of the solar wind flow and the period of the
oscillations. Results. We found that the estimated Strouhal numbers are very
small, and the typical value of $St\sim0.2$ would be extrapolated for size of
the halo larger than $\sim10^6$ km. Conclusions. Despite the vortex shedding
phenomenon has not been unambiguously revealed, the findings suggest that some
MHD instability process is responsible for the observed behaviour of cometary
tails, which can be exploited for probing the physical conditions of the
near-Sun region.
