GRAZ, Austria — A straight couple in Austria this month launch their bid to have a same-sex marriage.

"Registered partnerships" have been available to gays and lesbians in Austria since Jan. 1 thanks to a law passed Nov. 17, following a lengthy tug-of-war within the left-right coalition government. The partnerships have significant differences from marriages under Austrian law, some of which make them attractive to opposite-sex couples.

In debate over the law, the center-right Austrian People's Party insisted that same-sex couples not be allowed to use a hyphenated name, adopt children, have the right to fertility treatment or have a registry office ceremony. While both partnerships and marriages include key benefits such as tax breaks and access to social welfare, gay and lesbian rights campaign group LAMBDA says there are more than 72 differences in total between the two statuses.

Nevertheless, the new partnerships represent "progress," said Helmut Graupner, the Vienna-based lawyer leading the case on behalf of the opposite-sex couple, who are from Linz. "But if you introduce a new legal institution at the beginning of the 21st century then you can't make distinctions on the basis of gender."

"What the legislators did was to discriminate against same-sex couples wanting to get married as well as discriminate against opposite-sex couples, by not letting them form registered partnerships," Graupner added. "You can't be a little bit equal, just as you can't be a little bit dead or a little bit pregnant."

France's version of "marriage light" known as a "pacte civil de solidarite," or PACS, has been available to couples regardless of their gender since 1999 and has proved very popular. Of the 140,000 PACS formed in 2008, 92 percent were between couples with partners of different sexes.

"The couple I am representing have been together for 20 years and their children have already grown up," Graupner said. "They are not interested in adopting children or in medically-assisted pro-creation," which are areas where registered partnerships offer fewer rights than traditional marriage.

What has proved attractive to the couple, whose identity is so far being kept secret, is having a more "modern" form of partnership where divorce proceedings would be less time-consuming and alimony payments lower, Graupner said.

When both partners do not agree to divorce Austrian law currently allows the objecting partner to block separation for up to six years, one of the longest periods of all European jurisdictions. Registered partnerships halve this period.

"If you don't agree to a divorce within a traditional Austrian marriage then you have to go to court and prove whose fault it is that the marriage went wrong," said Kurt Krickler of HOSI, a gay and lesbian advocacy group. "People end up hiring private detectives to prove a betrayal they can use to prove their partner's 'guilt' in court."

And, instead of the mutual fidelity required by traditional marriage, Austrian registered partnerships only require a "mutual relationship of comprehensive trust," which Graupner said means the partners must only agree to be open and trustful toward one another.

There is no chance that the opposite-sex couple involved in the case will be able to register their partnership any time soon. They will first have to wait to have their application formally rejected and then challenge the law in the Constitutional Court, which could mean a wait of up to a year. The next stop would be the European Court of Human Rights (ECHR) in Strasbourg.

It would be 2015 before the ECHR could hear the case, Graupner said. It has previously rejected similar cases lodged against other EU member states, but "by that point there should be many more countries with full civil marriages open to same-sex couples and full sexual equality."

But Krickler is doubtful the case will succeed at either stage, saying equality is more likely to be achieved in Austria by the election of a less conservative government. A successful court battle could backfire, he said, with equality being delivered by saddling same-sex couples with the same strict divorce laws as in a traditional marriage.

For the moment Krickler is prepared to accept the status quo, "Everybody was surprised that the conservative party agreed to such a far-reaching piece of legislation." He was particularly pleased that registered partnerships allow non-EU citizens to gain the right to live and work in Austria when immigration is such a touchy subject for the right.

There has, so far, only been a trickle of same-sex couples registering their partnerships. "There is not much reason to do it unless you've got a partner from a third-party country," Krickler said. But he expects the numbers to grow a little as the weather becomes more conducive to celebration.