An Introduction to French Pronunciation

116 The Consonants in Detail: (III) /r/, /l/ and the Nasals
having someone correct them. In general, the dropping of the
/r/ is considered as more acceptable than the dropping of
the /l/, which is more likely to be regarded as substandard.
16.6 Voiceless /m/
A voiceless allophone of /m/, which can also be indicated by a
subscript [ ˚ ] (see 16.5.1), occurs when it follows the voiceless
fricative /s/, in words such as asthme [ast], spasme [spast].
This is a type of progressive assimilation (see chapter 18 and
especially 18.1.4 and 18.3). Apart from a few relatively infrequent
words such as these, it occurs in the ending -isme
of communisme [komynist], nationalisme [nasjbnalist],
cubisme [kybist], réalisme [realist], etc. (A pronunciation in
/izm/, i.e. with a voiced fricative and, consequently, a voiced
/m/ also exists but is less widespread.)
Similarly, a voiceless [t] occurs after the voiceless stop /t/
in rythme [ritt].