  We present SN 2019tsf (ZTF19ackjszs) and SN 2019oys (ZTF19abucwzt). These two
stripped envelope Type Ib supernovae suddenly showed a (re-)brightening in
their late light curves. We investigate this in the context of circumstellar
material (CSM) interaction with previously ejected material, a phenomenon that
is unusual among SE SNe. We analyse observational data, consisting of optical
light curves and spectra. For SN 2019oys we also have detections in radio as
well as limits from UV and X-rays. Both light curves show spectacular
re-brightening after about 100 days. In the case of SN 2019tsf, the
re-brightening is followed by a new period of decline, and the spectra never
show signs of narrow emission lines that would indicate CSM interaction. On the
contrary, SN 2019oys made a spectral makeover from a Type Ib to a spectrum
clearly dominated by CSM interaction at the light curve brightening phase. Deep
spectra reveal a plethora of narrow high ionization lines, including coronal
lines, and the radio observations show strong emission. The rather similar
light curve behaviour indicate CSM interaction as the powering source. For SN
2019oys the evidence for a phase where the ejecta hit H-rich material, likely
ejected from the progenitor star, is conspicuous. We observe strong narrow
lines of H and He, but also a plethora of high ionization lines, including
coronal lines, revealing shock interaction. Spectral simulations of SN 2019oys
show two distinct density components, one with density > 1e9/cm3, dominated by
somewhat broader, low ionization lines of H I, He I, Na I and Ca II, and one
with narrow, high ionization lines at a density about 1e6 /cm3. The former is
strongly affected by electron scattering. The evidence for CSM interaction in
SN 2019oys is corroborated by detections in radio. On the contrary, for SN
2019tsf, we find little evidence in the spectra for any CSM interaction.
