Investec has upgraded its rating for RBS from 'sell' to 'hold', playing down concerns about the upcoming Scottish referendum which has led to the stock's recent underperformance against the wider banking sector.The broker has lifted its target price from 340p to 355p."We believe any such event-specific weakness will unwind after 18 September as we anticipate a pro-Unionist vote," said analyst Ian Gordon.He acknowledged that the scale of the lead for the 'no' vote has narrowed, according to recent opinion polls."But we (in common with all UK bookmakers) still assume a rejection of independence by the Scottish people. As such, we believe it may yet prove costly to maintain a short position on RBS ahead of next week’s vote," Gordon said.The analyst also said that Investec has become slightly more positive on the bank's fundamentals and expects a gradual recovery across its 'core' retail and commercial operations.RBS' shares were up 0.2% at 344.8p by 10:22 on Wednesday.