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Short sellers can be interesting to watch, particularly in periods of market fluctuations. Here are five stocks that saw their short interest double between the January 15 and January 31 settlement dates, as the markets sold off. We have tracked the percentage change and included a note on days to cover or the percentage of each stock’s float, as well as a bit of color on each position.

BankUnited Inc. (NYSE: BKU) had a 126.8% short interest rise to 2.85 million shares, or 3.6% of the available shares. That was the highest level of short interest in at least a year, and the average daily volume was double that in the previous period. The company posted an earnings beat in the period. The days to cover remains more than two.

Boise Cascade Co. (NYSE: BCC) saw its short interest grow to 1.33 million shares from around 631,000 in the middle of the month. That gain more than erased a considerable decline in the previous period. About 4.2% of the float was held short. The days to cover dropped from more than four to about less than two. The wood products company appointed a new board member during the period.

Fortinet Inc. (NASDAQ: FTNT) saw a 144.6% jump in its short interest in late January to 6.61 million shares, or 4.8% of the total float. That is by far the largest number of shares short in the past year, and the average daily volume in the period was nearly double as well. The network security company posted better-than-expected results in January. The days to cover remained about one.

General Dynamics Corp. (NYSE: GD) saw the biggest rise in short interest among these companies, a whopping 380.6%. That was more than 15 million shares, or 4.7% of the company’s float. The defense contractor reported better-than-expected results during the period. It would take almost four days to close out all short positions.

Veeco Instruments Inc. (NASDAQ: VECO) saw its short interest surge by 104.4% to more than 7.60 million shares, or 19.9% of Veeco’s float. The CFO sold more than 58,000 shares during the period. The last time the semicondictor company’s short interest rose above 7 million shares was back in June of 2013. It now would take about 15 days to cover all short positions.