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

The circumstellar envelopes of the Cepheids L Car and RS Pup - Comparative study in the infrared with Spitzer, VLT/VISIR and VLTI/MIDI