The Vowels /ø/ and /œ/ (e.g.: feu and seul)

Features of /ø/ and /œ/

The vowels /ø/ and /œ/ are characterized by the following features:

/ø/ is mid-closed, front, rounded and oral, e.g.: peu [pø] , eux [ø] , deux [dø] .

/œ/ is mid-open, front, rounded and oral, e.g.: seul [sœl] , coeur [kœʁ] , leur [lœʁ] .

As you can see, these vowels are identical, except that the tongue is a little higher for /ø/ than it is for /œ/.

Articulation of /ø/ and /œ/

Like all French vowels, /ø/ and /œ/ are short and tense. Since they are both rounded vowels, the lips must be rounded AND advanced during articulation. In order to arrive at a correct pronunciation of these vowels, it is advisable to begin with their unrounded counterparts, e.g.:

/e/ → /ø/

des [de] → deux [dø]

fée [fe] → feu [fø]

/ɛ/ → /œ/

sel [sɛl] → seul [sœl]

père [pɛʁ] → peur [pœʁ]

Distribution

The vowels /ø/ and /œ/ tend to follow the Loi de position, i.e.: /ø/ is usually found in open syllables (affreux = [afʁø]) and /œ/ is usually found in closed syllables (seul = [sœl]). Note also that, as far as the Loi de position is concerned, the semi-consonant /j/ functions as a consonant. In other words, it is preceded by /œ/, e.g.: feuille [fœj] , orgueil [ɔʁgœj].

Exceptions to the Loi de position involve the presence of /ø/ in closed syllables. For the most part, spelling conventions will help (see below).

There are very few minimal pairs that are distinguished only by the alternation between /ø/ and /œ/. They do exist, however, e.g.: jeune/jeûne ([ʒœn] , [ʒøn]  ), veulent/veule ([vœl], [vøl]).

Spelling of /ø/

The vowel /ø/ is represented by the letters "eu" or "oeu" in certain contexts. Namely:

a) in open syllables, e.g.: feu [fø], affreux [afʁø]

b) in syllables closed by /z/ or /t/, e.g.: menteuse [mãtøz], neutre [nøtʁ].

Note that the past participle of avoir, i.e.: eu, is an exception since it is actually pronounced /y/.

Spelling of /œ/

The vowel /œ/ is represented by the letters "eu" or "oeu" followed by a pronounced (semi-)consonant other than "z" or "tr", e.g.: peur [pœʁ], veulent [vœl], deuil [dœj], aveugle [avœgl]. There are some instances where /œ/ is represented by "ue", e.g.: accueil [akœj]. Here, the "u" comes first to indicate that the "c" is pronounced /k/ and not /s/ (see here).

/ø/ and /œ/ in unaccented position

When "eu" occurs at the beginning or in the middle of a word, the vowel is usually /ø/, e.g.: Europe [øʁɔp], jeudi [ʒødi]. That said, if the syllable is closed, use /œ/, e.g.: seulement [sœl-mã].

• The word monsieur is pronounced [møsjø] 
• The singular, l'oeuf, is pronounced with /œ/, e.g.: [lœf] while the plural, les oeufs, is pronounced with /ø/, i.e.: [lezø] (this is in keeping with the Loi de position).
• The word oeil is pronounced [œj], while yeux is pronounced [jø] (in with the Loi de position).
• The second syllable of the word déjeuner has a mid-open front rounded vowel [deʒœne].
• When pronouncing the sequence [œj], make sure you do not replace it with the English diphthong [oi] (e.g.: the one in the word boy).