The below quote is an example that Longman has provided to illustrate the usage of the adjective "incredulous". I believe in this example, "incredulous" in used as an adverb. Please help me understand how it can possibly be acceptable.
"You sold the car?" she asked, incredulous.
All I can guess is that the original sentence was something like "she asked, with an incredulous manner" before it underwent a series of mysterious ellipses!