Arbitrage-related short interest continues to grow in shares of  Cleveland's Charter One Financial. 
As of Sept. 15, nearly 2.93 million shares of the thrift company had  been sold short, an increase of more than 369,000 shares from a month   earlier, according to data from the National Association of Securities   Dealers.     
  
Arbitragers keen to lock in a risk-free gain based on the share exchange  ratio in Charter One's deal to buy Firstfed Michigan Corp. are the   overwhelming reason for the increase, according to analysts.   
"They look for a guaranteed return by going short in shares of the buyer  and long in those of the seller,' said analyst Joseph A. Stieven of Stifel,   Nicolaus & Co., St. Louis.   
  
Shareholders of Firstfed Michigan, based in Detroit, are set to receive  1.2 shares of Charter One for each of their Firstfed shares when the   acquisition is completed.   
Mr. Stieven said that besides the deal, he could imagine no reason for  shorting the shares of Charter One. The thrift has excellent fundamentals,   he said, and added "we like their Firstfed deal."   
The rise in Charter One short interest, which occurred through the  summer, put it in third place in short activity among banking and thrift   stocks traded on the Nasdaq market system.   
  
In short-selling, investors borrow shares and then sell them with the  prospect of profiting by repaying the lender with cheaper shares. Short   interest is the total number of shares of a company's stock sold short.   
Traditional short-selling is essentially a bet that a stock's price will  fall or at least underperform the overall stock market, but short activity   has also been increasingly employed as a part of investment ploys like   merger-related arbitrage.     
Both versions of short-selling are apparently at work in shares of U.S.  Bancorp, where short interest jumped another million shares between mid-   August and Sept. 15, to more than 8.1 million.   
The Portland, Ore., banking company has by far the highest level of  short interest among Nasdaq bank stocks. 
  
U.S. Bancorp's deal to acquire Boise, Idaho-based West One Bancorp is  the biggest reason for the increased short activity, but the transaction   has also been sharply criticized by some analysts as too high priced for   the benefits it will provide.     
Indeed, shares of bank acquirers have been performed relatively less  well than banks overall this year, making them candidates for short   activity.   
For a well-traded stock, U.S. Bancorp also carries an extremely high  short-interest coverage ratio - 17.4 days. This means short-sellers of the   stock would need that many days, at the stock's recent typical daily   volume, to buy enough shares to repay lenders and thus "cover" their short   positions.       
As of Sept. 15, Charter One had the longest coverage ratio -24.5 days.  In third place was First Commerce Corp., New Orleans, at 12.8 days. 
There were also big decreases in short activity in some Nasdaq bank  issues, notably Huntington Bancshares of Columbus, Ohio, whose shares sold   short fell by over 244,000 in the month through Sept. 15.   
In other bank stock news, Anthony R. Davis of Dean Witter Reynolds Inc.  downgraded Wachovia Corp. to "neutral" from "accumulate." The analyst   noted that stock had risen 19% in value this year.   
Mr. Davis cut his 1995 earnings estimate for the North Carolina bank by  10 cents to $3.40 per share and his 1996 forecast by five cents to $3.75   per share. He noted the bank is planning large expenditures for new   technology.     
Analysts at Keefe, Bruyette & Woods Inc. upgraded both BankAmerica  Corp., San Francisco, and Mellon Bank Corp., Pittsburgh, to "buy" from   "attractive."   
Analyst Francis X. Suozzo of S.G. Warburg & Co. upgraded shares of  Household International to "buy" from "hold". He cited the finance   company's strengthened balance sheet and said it can sustain earnings per   share growth of 20% through 1997.