The three original comic strips from Bill Watterson's surprise guest stint last month on Pearls Before Swine will be displayed this week at Comic-Con International before they're sold at auction Aug. 8, with proceeds benefiting The Michael J. Fox Foundation for Parkinson’s Research.

The collaboration, which came at the suggestion of the Calvin and Hobbes creator, marked Watterson's return to the comics page after a 19-year absence. Pastis teased readers that the week's storyline would "contain a mind-blowing surprise,” but didn't reveal what it was. Nevertheless, some fans quickly uncovered clues that some of the strips were ghost-drawn by Watterson.

“Any time original Bill Watterson comic art shows up for auction it’s a big deal,” Todd Hignite, vice president at Heritage Auctions, said in a statement. “His collaboration with Stephan Pastis was an unexpected treat for his millions of fans. Now, thanks to this auction, fans will get to take the original art home while raising money for a great cause.”

At Watterson's request, the art is being sold on behalf of Team Cul de Sac, the charity founded by Chris Sparks to honor Cul de Sac creator Richard Thompson, who has Parkinson’s disease. The June 5 strip has already reached $11,000 in online bidding, with the other two close behind at $9,500 and $5,500.

A painting by Watterson of one of Thompson’s characters sold in 2012 for $13,000 as part of a benefit auction for Team Cul de Sac. A rare original 1986 Sunday installment of Calvin and Hobbes, drawn and hand-colored by Watterson, fetched $203,150 the same year.