  [Abridged] - Context: Circumstellar envelopes (CSEs) around Cepheids are
particularly interesting as they could impact the Cepheid distance scale, and
imply stellar mass loss. Aims: Our goal is to establish the spatial and
spectral properties of the CSEs of L Car and RS Pup. This is done through a
parametrization of the envelopes in terms of fractional flux (with respect to
the star) and angular size. - Methods: We retrieved archival Spitzer images of
the two stars (3.5-70 mic), and obtained new imaging with the VLT/VISIR camera
in BURST mode (8.6-11.9 mic), as well as interferometry with VLTI/MIDI (8-13
mic). This combination allows us to probe the envelopes over arcminute to
milliarcsecond scales. - Results: The CSE of RS Pup is resolved at 24 and 70
mic by Spitzer, and around 10 mic by MIDI and VISIR. The compact envelope of L
Car is resolved only in the VISIR and MIDI observations. However, the
properties of the IR excesses differ considerably : a warm component is present
around both stars at a spatial scale of a few 100 to a few 1 000 AU, but RS Pup
presents in addition a large (several 100 000 AU) and cold (~40 K) dusty
envelope. - Conclusions: We propose that the reflection nebula surrounding RS
Pup has an interstellar origin, while the warm CSEs of both stars were created
by ongoing stellar mass loss. Such warm circumstellar envelopes are probably
common around Cepheids.
