O Kossoy Sisters, where art thou been?

And even while sitting across a table from them at a Harvard Square coffee shop, it's difficult to keep track of which one is which.

Until "O Brother, Where Art Thou?" came out two years ago with their version of "I'll Fly Away," most music fans didn't keep track of either one.

It's been a long time between albums for the Kossoy Sisters; nearly a half-century, in fact, since the 1956 release of "Bowling Green," their only other release to date (although Irene also recorded an album with ex-husband Tony Saletan in the late '60s).

Recorded when the sisters were only 17, "Bowling Green" has rarely, if ever been out of print since its original release on Tradition Records, and it's easy to understand why.

The sisters' singing is perhaps best described as "haunting," with Irene handling most of the lead vocals and Ellen nailing impossibly high harmonies with apparent ease, accompanied by the spare instrumentation of their own guitars and guitarist/banjo player Erik Darling.

It's a pleasant surprise to discover that "Hop On Pretty Girls" - released in November on the Living Folk label - sounds pretty much how one would expect a Kossoy Sisters album to sound in 2003.

The instrumentation is a bit more varied than on the first album, and a couple of original numbers have made the cut this time around (as well as a re-recording of the first album's "The Wagoner's Lad"), but the sisters' vocals happily show little evidence of the passing years. The singing is clearly the work of individuals who are older than those who made the first album, but any fan of "Bowling Green" will immediately recognize the voices.

So why did it take 46 years to make a second album?

According to Ellen, "Part of it is inertia. Part of it is the fact that we lived over a thousand miles from each other up until almost four years ago. We'd get together maybe once or twice a year and play music, but most of the time we were apart, so it just didn't seem like an opportune time to do it."

Born in New York City in 1938, the Kossoys were exposed to music at an early age.

"We always sang together," says Irene. "Ever since we were able to talk, we were singing. Our mother's sister lived with us and the two of them were always singing together in harmony, so we kind of figured that out fairly quickly when we were about six or so. We started singing these songs when we were around 15. We liked The Weavers -- they were popular -- and with our older sister we used to go to what were then called hootenannies, which were different from hootenannies in the '60s. They were more political, not as mainstream."

"We went to a summer camp when we were 15, and it was right near where Pete Seeger lived. So he would come down and bring people with him. So we just started getting interested in playing the guitar. Then we started going to Washington Square in the Village every Sunday, just hanging out and learning songs."

The Greenwich Village of the '50s and early '60s was a hotbed of writers, musicians and leftist political discourse, and the sisters met dozens of artists who were either already big names in folk music or soon would be: Dave Van Ronk, Ralph Rinzler, Mike Seeger, John Cohen and Mary Travers among them.

One of the musicians they had met during this period was Darling, who in 1958 would replace Pete Seeger in The Weavers and in 1962 scored a substantial hit as a member of the Rooftop Singers with "Walk Right In." In 1956, however, Darling was the sisters' occasional live accompanist and is the only other musician heard on "Bowling Green."

"We were hanging around Washington Square, and a woman named Lucia Walker came over to us and asked us if we would like to do some concerts," Irene says. "After the off-Broadway theatres would go dark, they would have midnight concerts. So we started doing a number of these concerts, some with Erik Darling. So then Paddy Clancy of the Clancy Brothers was starting up Tradition Records and he approached us and asked if we would do a record."

"We recorded a whole lot of stuff, and they picked which ones they liked," says Ellen. "I remember that there were some we sang that they didn't put on there."

"I think it did well," says Ellen. "Times were different then. There wasn't as much out there as there is now. It's never really been out of print. It would come back into print, or people would find it in used (record) stores. Financially, it didn't do that well for us, but it sold a lot of copies, and a lot of people knew of it."

It was also during this period that the sisters got to know Woody Guthrie, whose "Belle Starr" is covered on the new album. Though ill with Huntington's Chorea and retired from performing, Guthrie at this time was still capable of leaving the hospital for short periods of time and first met the Kossoys when they performed at a concert of his songs.