With Premier Oil (LSE: PMO) having risen by 46% since the start of the year, many investors may feel that it is too late to buy a slice of the oil producer. After all, the price of oil has already soared to around $50 per barrel and investor sentiment towards the wider oil sector has improved dramatically.

However, there is still considerable upside potential on offer for investors in Premier Oil. That’s because the price of oil could increase significantly in the long run as demand from the emerging world rises. Certainly, in the short run it could come under pressure but for long term investors, oil remains a very appealing industry in which to invest.

Moreover, Premier Oil still trades on a very appealing valuation even after its recent share price rise. For example, it has a price-to-book (P/B) ratio of 0.7, which indicates that capital gains are still very much on the cards in the long run and that now could be an excellent opportunity to buy.

Similarly, shares in Lonmin (LSE: LMI) have soared since the turn of the year. In fact, they have risen by a whopping 129% year-to-date, but there could be much more to come. That’s not only because there is scope for commodity prices to rise, but also because Lonmin seems to have a sound strategy through which to increase its earnings.

For example, it is seeking to reduce costs and become increasingly efficient. And with Lonmin having raised capital last year, it appears to have the financial firepower through which to deliver improved financial performance in the coming years. In fact, Lonmin is forecast to return to profitability in the next financial year, which has the potential to significantly improve investor sentiment in the stock.

Certainly, Lonmin’s forward price-to-earnings (P/E) ratio of 68 may seem to be excessively high, but when its P/B ratio of around 0.5 is factored in, there seems to be scope for further capital gains. While they may not be as impressive as those achieved in the last five months, they could still be well ahead of the wider index over the medium to long term.

Meanwhile, shares in Swallowfield (LSE: SWL) have risen by 24% today after it announced the acquisition of The Brand Architekts Limited, as well as a placing to raise £8.6m. The deal seems to be a logical one for the personal care and beauty product specialist, with Brand Architekts having the potential to bring critical mass to Swallowfield’s owned brand portfolio, as well as adding a proven management team. In addition, the deal looks set to be immediately earnings accretive and with the placing being oversubscribed, it seems popular among investors.

Clearly, after such a jump in share price many investors will feel that a pullback is on the cards. However, with Swallowfield trading on a price-to-earnings growth (PEG) ratio of just 0.3 prior to today’s update, it appears to still offer capital gain potential. And with today’s trading update being in-line with expectations, now could be a good time to buy Swallowfield.

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Peter Stephens has no position in any shares mentioned. The Motley Fool UK has no position in any of the shares mentioned. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.