Brexit has left these FTSE 100 stars looking far too cheap!

It comes as little surprise that Britain's shock decision to exit the EU is causing investors' hearts to skip a beat or two.

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But while much of the FTSE 100 (INDEXFTSE: UKX) has recovered ground since 23 June -- indeed, the index spiked to 11-month peaks last week -- many of Britain's big-caps are still looking oversold.

Soaring star

Although not a member of Britain's elite index, budget flyer Ryanair(LSE: RYA) is still one giant stock going far too cheap, in my opinion. At first glance investor worries can be expected -- after all, spending on holidays is one of the first things to collapse as economic cooling hits.

Still, I expect Ryanair's route expansion programme to keep driving revenues growth. The Irish airline saw passenger numbers leap 11% during June, to 10.6m, even as strikes by French air traffic controllers weighed. Besides, Ryanair is likely to benefit from rising custom as travellers opt for cheaper tickets.

So while the firm isn't without risk, I believe a prospective P/E rating of 10.1 times is more than decent value.

Ready to motor

It's fair to say that car parts builder GKN(LSE: GKN) has had a year to forget. First battered by the Volkswagen emissions scandal in 2015, GKN's share price has also been pressured by fears of slowing demand in the key Chinese market.

And like Ryanair, investor appetite for GKN has fallen further since the Brexit vote, with stock-pickers fretting over diving 'big ticket' purchases in the UK and beyond.

However, I believe the long-term outlook for the car market remains robust, as does the revenues potential from the civil aircraft segment, another critical area for GKN. A backcloth of rising populations and wealth levels in emerging regions is likely to propel demand for cars and planes in the years ahead.

Consequently I reckon GKN is great value given its ultra-low forward P/E rating of 9.8 times.

Paper tiger

The essential nature of Mondi's(LSE: MNDI) goods makes it a great pick for those seeking to Brexit-proof their portfolio.

The paper play provides a variety of goods for multiple sectors, from food packaging and refuse bags to envelopes. And Mondi's global expansion drive provides extra peace of mind -- the firm sucked up Turkish consumer packaging provider Kalenobel just last month.

Many investors have cottoned on to Mondi's terrific defensive qualities, and the stock recently struck its highest since December. But a P/E rating of 12.9 times still makes it brilliant value for money, in my opinion.

'Pru' power

Investors should quite rightly be concerned about the impact of Brexit on financial services. Prudential's(LSE: PRU) M&G investment arm was forced to suspend trading at its £4.4bn property fund due to rising redemption in recent days.

Allied to this, the impact of a potential cooldown in The Pru's British insurance division could cause significant earnings headwinds too.

Still, I'm convinced the company's beefy presence in the US and Asia should help to mitigate these problems and drive growth. Prudential has famously made expansion in Asia one of its key priorities, and with good reason. Operating profit here leapt 17% in 2015.

So while Prudential isn't without its fair share of danger, I reckon a forward P/E rating of 10.9 times could prove a shrewd level at which to tap into the firm's hot long-term prospects.

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