Fans of both Lady Gaga and Kendrick Lamar got at least a partial treat yesterday when the previously unreleased track "Partynauseous" leaked online. It might seem odd that a collaboration between one of the biggest pop stars in the world, plus one of the most in-demand rappers in the game, didn't make it onto an album...but you might be surprised how many Gaga collaborations have yet to see the light of day (or have only done so in an unofficial context). We're not talking about the pre-Fame Gaga guest spots, such as her appearance with New Kids on The Block...we're talking about music that's yet to drop, including possible collaborations with stars such as Paul McCartney and Cher. Here's what we know and, in some cases, what we've heard:

Paul McCartney

A furor started online when some snarky Twitter users began the hashtag "#WhoIsPaulMcCartney" after the Beatle collaborated with Kanye West on "Only One"...which was obviously a joke. After all, Lady Gaga obviously discovered Paul McCartney. In all seriousness however, there is one or multiple recordings featuring the Fame Monster herself collaborating with the world's most influential, living musician. The good news: This song (or these songs) have only not been released because Macca is still working on the project.. "Had a beautiful session with Sir Paul McCartney and friends," Gaga posted on Instagram with a photo of herself among a group of musicians (one of which is Pearl Jam guitarist Mike McCready). "Working on one of his many secret projects! Killer musicians, vibe, and lots of laughs." It's somewhat misleading of us to suggest that this collaboration had been "shelved," when it's more likely than not to see a release in the near future. Let's just hope that future is nearer than farther.

[EDITOR'S NOTE: News broke today via The Guardian today that this session was for the soundtrack to High in The Clouds, the animated film version of the children's book McCartney wrote during 2005. According to the new reports, both Macca and Gaga will appear on one track together.]

Cher

It will come as as surprise to roughly no one that Lady Gaga is a huge fan of Cher and therefore had no problem gifting a song—"The Greatest Thing"—to her during 2012. Well, kind of. There are very few official reports on what the fate of the song was, but Cher initially suggested that the song would appear on her 2013 album Closer To The Truth, possibly as a solo performance or as a duet featuring the songwriter as well. The details get sketchy from there...some suggested that Gaga insisted that the song be released a joint single, versus a "Cher featuring Lady Gaga" setup. Others suggest that the younger pop star simply didn't like the finished project after both had done their work. Either way, it didn't end up on the album. You can easily find "The Greatest Thing" on YouTube in varying levels of quality...here's one that sounds pretty good. Bummer it didn't get a wider release.

The Midway State

Lady Gaga was the hot item during 2009-her Fame had blown up and The Fame Monster EP was about to blow up even further. Then would have been a good time to release any collaborations you had with the young performer, especially if you yourself were a group struggling to gain traction in the U.S. music market. We're not sure exactly when Canadian indie rockers The Midway State teamed up with the pop star for their cover of "Don't Give Up" by Peter Gabriel, but somehow it found its way online during 2009. The original single, although appearing on Gabriel's album, was actually a duet between the former Genesis member and Kate Bush, and Gaga jumped on this collaboration explicitly to promote the latter performer. She did the song "so that young people would hear and learn something about Kate Bush."

Azealia Banks

This is definitely one of the more interesting collaborations we've heard of in recent years, by far the one we're most curious about on this list and, unfortunately, the only one that doesn't seem to have any leaked audio to go along with it. It was reported way back when that Gaga and Azealia Banks, easily two of the most independently-minded members of the music industry, were working on a collaboration, either for Gaga's Artpop or for Banks' Broke With Expensive Taste. As is usually the case with Banks collabs, things broke down and the rapper took to Twitter to accuse the pop star of stealing a) her mermaid aesthetic and b) the song "Red Flame" from Banks. This touched off all sorts of speculation: For one, Gaga had yet to release a tracklist for Artpop...so no one was really sure where Banks was getting her facts. When Artpop did drop, there was no track "Red Flame." Nor did Banks include the track on her album when it finally dropped last year. All we know is that at some point both Banks and Gaga had initially planned on working together on a song titled "Red Flame." Where that audio is now, if it indeed exists as Banks claims, is a mystery.

Darkchild

Between Lady Gaga and Beyoncé, it was easy to forget that anyone else had anything to do with the 2009 single "Telephone." The third character that deserves a healthy amount of credit is Rodney "Darkchild" Jerkins, who produced the track and crafted its beat. As was the case with many producers of the day (Timbaland et al), Jerkins couldn't resist making an appearance as a vocalist as well alongside his star. Therefore he teamed up with Gaga for "Reloaded," a track that featured both his production and lyrics as a rapper. It was initially reported that the song would serve as the B-side to "Telephone" but that never panned out (the U.S. 7" release featured a Passion Pit remix instead), nor did the song ever make it onto any of Gaga's releases. As much as Darkchild himself might disagree, we can't fault Gaga for leaving this one be.