Is letter U pronounced in French between G and A or O?

Question

There are a few words in French with letter U which stands after G and before either A or O. Examples include words such as conjuguais and conjuguons (forms of the verb "conjuguer" — "to conjugate").
I wonder if the letter U is pronounced in these cases or is silent.
All my textbooks say that U in French is silent after G before E or I. They don't tell anything about A or O.

Answer 1

Quick answer: in the two examples you provide, the U's are not pronounced.

There are however cases where they would be (several additions following jlliagre’s comment below):

• [w]: guano [ɡwano] — iguane [iɡwan] — jaguar [ʒaɡwaʁ] — Guadeloupe, Guatemala, Nicaragua, Paraguay, Uruguay and their derivatives
  
• [ɥ]: aiguille (needle) [eɡɥij] — linguiste [lɛ̃ɡɥist]
  
• [y] (typical U sound of French): aigu (feminine aiguë, acute) [eɡy]

With the verb conjuguer (as well as others in -GUER), the U is maintained in all conjugations, regardless of the letter that would otherwise be following the G. It is exceptional and unusual enough to deserve a note in most conjugation reference books.

Following comments to this post, here’s a non-exhaustive list of verbs in -GUER that also maintain the U in all conjugations:

dialoguer
distinguer
monologuer
narguer
naviguer
zigzaguer
Also divaguer and tanguer (although Larousse conjugation book puts no special note on these two, I am not accustomed to see ‘je divagais’, but rather ‘je divaguais’)

Also worth mentioning is the verb arguer, whose U is actually pronounced: [aʁɡɥe] (also commonly pronounced /aʁɡye/), and whose conjugation seem reasonably stable on the various online conjugation tools: like here or there, though Larousse have a choice for many persons and tenses, that is, many conjugations are also proposed with diaereses (j’arguë, which I suppose could also become j’argüe since the spelling reform of 1990).

And finally, the case proposed by Chambaron: two words pronounced the same ([baɡaʒ]), but with a different meaning depending on the presence or not of the U after the G: bagage (luggage) and baguage (bird-banding).