uə actualShe objected that these
ne sont pas des diphtongues (voyelle se modifiant en cours d'émission, prononcée en 1 seule syllabe) mais des voyelles non phonémiques /i/ et /u/ suivies d'un schwa /ə/, appartenant à 2 syllabes contiguës mais distinctes, malgré l'absence de consonne intermédiaire.Longman asked me to draft a reply.
[are not diphthongs (a vowel modified in the course of production, pronounced in one single syllable), but non-phonemic vowels /i/ and /u/ followed by a schwa /ə/, belonging to two adjacent but distinct syllables, despite the absence of any consonant between them.]
In LPD I have a whole-page panel discussing what I call Compression. You could also call it Varisyllabicity. The point is that some English words are varisyllabic: the number of syllables they contain is variable. Thus actual can be pronounced as two syllables or as three; peculiar can be pronounced as three syllables or as four.
The sounds represented as uə and iə here can be, and frequently are, pronounced in a single syllable. Alternatively, particularly in slow, careful, overarticulated speech, they can be pronounced in two syllables.
If "compressed" into a single syllable, they are diphthongs. Daniel Jones treated them as a particular type of diphthong, rising diphthongs. I have sometimes called them crescendo diphthongs. (It may be difficult or impossible to distinguish them auditorily from the corresponding sequences of semivowel plus schwa, wə or jə.)
I would think you get the same alternation in French, for example in the name Louis, which is usually monosyllabic [lwi] but can also be said disyllabically as [lui].