Aaron Hernandez's Fiancee -- Judge Grants Immunity ... In Exchange for Testimony

Aaron Hernandez's fiancee has been granted immunity in exchange for her testimony in the former NFL star's murder trial ... but the extent of her protection is a big question mark.

The judge in the Odin Lloyd murder trial just announced an "endorsement for immunity to Shayanna Jenkins."

In the past, prosecutors have accused Shayanna of disposing of possible evidence at Hernandez's request ... after Odin's murder.

Now here's the mystery -- the judge has sealed the record ... so it's unclear how much protection Shayanna will receive in exchange for spilling her guts.

According to reports, prosecutors claim Shayanna has already been busted lying to the grand jury 29 times ... so, in theory, the "immunity" could merely protect her from perjury charges.

On the other hand, it's possible the immunity could sweep more broadly ... and protect her from more serious charges if she incriminates herself on the stand.