The Fannie Mae and Freddie Mac crisis has been a nightmare for some US pension funds and a blessing for others. On the one hand there are the funds that have seen a fall in the value of the stocks they hold in the two government-sponsored enterprises (GSEs), whose public mission is facilitating a secondary market in home mortgages - purchasing mortgages from the lenders who originate them, then selling them in the form of securities.

Since last summer, when the sub-prime crisis started to unfold, the GSEs lost most of their market capitalisation. Fannie Mae's stocks tumbled from $70 (€44) to $10 and less, while Freddie Mac's dived from $60 to $6. In a few days in mid-July the losses were around 25-30% because of rumours that their situation was so serious as to require government intervention to avoid bankruptcy.

On the other hand, the funds that hold the GSEs' bonds - which include the majority of the US retirement systems - are happy because a government intervention could only strengthen the agencies' credit ratings and their ability to pay on their debt. In fact their bonds' prices rose after the rumours, and the spread between the agencies' bond yields and yields on comparable US Treasury bonds decreased in mid-July.

Pension funds, central banks and other institutional investors hold $5.2trn in debt sold by the two agencies. Their bonds have long been popular with investors, notwithstanding increased concerns about their weak capital position and high financial leverage. The implicit US government guarantee has granted them an AAA rating, lowering their cost of borrowing and allowing pension funds to buy their bonds without problems.

In addition, their bonds' yields have always offered a little more basis points than US Treasuries. Now that the US government guarantee has become quite explicit, pension funds and other investors are even happier.

The perspective is different for stockholders such as the Ohio Public Employees Retirement System (OPERS), which is a long-time foe of Freddie Mac. OPERS's 920,000 members work for about 3,700 government agencies in Ohio. Its assets exceed $77bn and it is the 14th largest US fund. In 2006 it won a $410m settlement in a class action where it was the lead plaintiff. The suit accused Freddie Mac of misrepresenting its financial position to shareholders after the news in 2003 that from 2000 to 2002 it had manipulated records to meet Wall Street earnings expectations and it had to restate $5bn of earnings, one of the largest corporate restatements in US history.

"Freddie Mac admitted having trouble with internal controls and lacking transparency," says Chris Geidner, counsel to the Ohio attorney general's office that had filed the lawsuit on behalf of OPERS.

The $410m is being distributed to class members who bought Freddie Mac stock between 15 July 1999 and 20 November 2003.

But OPERS is again the lead plaintiff in a class action against Freddie Mac which was filed in January. It alleges that the GSE did not properly advise stockholders about its sub-prime exposure. It claims: "In November 2007, Freddie Mac was finally forced to disclose the truth about its significant exposure to the sub-prime crisis when it announced that its sub-prime exposure had resulted in a record $2bn loss. As a result of these revelations, the price of Freddie Mac common stock plummeted by 29%, with common stock shareholders losing over $6.6bn in market capitalisation in one day."

This time the class action suit is on behalf of investors who purchased common stock between August 2006 and 23 November 2007. During this period OPERS lost $27m because of Freddie Mac's nose-diving stock.

Ohio attorney general Marc Dann, who filed the latest lawsuit on behalf of OPERS, claimed Freddie Mac concealed its heavy sub-prime investments and "secretly and intentionally participated in one of the largest housing investment deceptions in modern US economic times".

The GSE should file its rebuttal this month and then the class action may move on.

Even before the sub-prime crisis, Fannie Mae come under fire for accounting problems. In 2004 a Congress investigation accused its top managers of deliberately misrepresenting net earnings to maximise their bonuses. But, so far at least, it has not been the defendant in a class action initiated by pension funds.